Earth-boring (or drilling) bits are commonly employed for oil and natural gas exploration, mining and excavation. Such earth-boring bits may have fixed or rotatable cutting elements. FIG. 1 illustrates a typical rotary cone earth-boring bit 10 with rotatable cutting elements 11. Cutting inserts 12, typically made from a cemented carbide, are placed in pockets fabricated on the outer surface of the cutting elements 11. Several cutting inserts 12 may be fixed to the rotatable cutting elements 11 in predetermined positions to optimize cutting.
The service life of an earth-boring bit is primarily a function of the wear properties of the cemented carbide inserts. One way to increase earth-boring bit service life is to employ cutting inserts made of materials with improved combinations of strength, toughness, and abrasion/erosion resistance.
As stated above, the cutting inserts may be made from cemented carbides, a type of cemented hard particle. The choice of cemented carbides for this application is predicated on the fact that these materials offer very attractive combinations of strength, fracture toughness, and wear resistance (i.e., properties that are extremely important to the efficient functioning of the boring or drilling bit). Cemented carbides are metal-matrix composites comprising carbides of one or more of the transition metals belonging to groups IVB, VB, and VIB of the periodic table (Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W) as the hard particles or dispersed phase, and cobalt, nickel, or iron (or alloys of these metals) as the binder or continuous phase. Among the different possible hard particle-binder combinations, cemented carbides based on tungsten carbide (WC) as the hard particle, and cobalt as the binder phase, are the ones most commonly employed for earth-boring applications.
The properties of cemented carbides depend upon, among other properties, two microstructural parameters, namely, the average hard particle grain size and the weight or volume fraction of the hard particles or binder. In general, the hardness and wear resistance increases as the grain size decreases and/or the binder content decreases. On the other hand, fracture toughness increases as the grain size increases and/or the binder content increases. Thus there is a trade-off between wear resistance and fracture toughness when selecting a cemented carbide grade for any application. As wear resistance increases, fracture toughness typically decreases and vice versa.
FIGS. 2A-2E illustrate some of the different shapes and designs of the cemented carbide inserts typically employed in rotary cone earth-boring bits. Cutting inserts for earth-boring bits are typically characterized by the shape of the domed portion 22A-22E, such as, ovoid 22A (FIG. 2A), ballistic 22B (FIG. 2B), chisel 22C (FIG. 2C), multidome 22D (FIG. 2D), and conical 22E (FIG. 2E). The choice of the shape and cemented carbide grade employed depends upon the type of rock being drilled. Regardless of shape or size, all inserts have a dome portion, such as, 22A-22E and a body portion 21. The cutting action is performed by the dome portion 22A-22E, while the body portion 21 provides support for the dome portion 22A-22E. Most, or all, of the body portion 21 is embedded within the bit body or cutting element, and the body portion is typically inserted into the bit body by press fitting the cutting insert into a pocket.
As previously stated, the cutting action is primarily provided by the dome portion. The first portion of the dome portion to begin wearing away is the top half of the dome portion, and, in particular, the extreme tip of the dome portion. As the top of the dome portion begins to flatten out, the efficiency of cutting decreases dramatically since the earth is being removed by more of a rubbing action, as opposed to the more efficient cutting action. As rubbing action continues, considerable heat may be generated by the increase in friction, thereby resulting in the insert failing by thermal cracking and subsequent breakage. In order to retard wear at the tip of the dome, the drill bit designer has the choice of selecting a more wear resistant grade of cemented carbide from which to fabricate the inserts. However, as discussed earlier, the wear resistance of cemented carbides is inversely proportional to their fracture toughness. Hence, the drill bit designer is invariably forced to compromise between failure occurring by wear of the dome and failure occurring by breakage of the cutting insert. In addition, the cost of inserts used for earth-boring applications is relatively high since only virgin grades of cemented hard particles are employed for fabricating cutting inserts for earth-boring bits.
Accordingly, there is a need for improved cutting inserts for earth-boring bits having increased wear resistance, strength and toughness. Further, there is a need for lower cost cutting inserts.